To elucidate certain aspects of enamel organ function, the movement of electron dense tracers through the enamel organ of rat incisor is studied with the electron microscope. Horseradish peroxidase is used to study the properties of the extracellular compartment of the enamel organ, such as its subdivision into smaller compartments and flow patterns of extracellular fluid. It is hoped furthermore that the tracer will help to identify the movement of material across the enamel- ameloblast interface, either into or out of the enamel. This study may thus contribute to a more enlightened description of secretion and maturation processes than has been possible up to now.